The present invention relates to current amplifiers.
Current amplifiers are widely used in signal detection/processing systems. For instance, sensors may provide very weak currents proportional to the physical variables acquired. Such current signals may need to be amplified for further processing in electronic circuits.
Two approaches are commonly used.
The first approach consists in using current mirrors. The amplification is performed using large transistors aspect ratios, Dell""Ova et al. (U.S. Pat. No. 5,867,066) describe a circuit which uses current mirrors in one embodiment of their invention. Unfortunately, such designs suffer from great limitations; their implementation requires large silicon surfaces to accommodate large transistors. Such large silicon surfaces produce consequently large parasitic capacitances, increasing delay and power dissipation. Moreover, such current amplifiers usually can not function correctly at weak-current levels.
A second approach consists in employing cascaded transimpedance and transconductance amplifiers. This approach may provide a higher current gain than the first approach. Unfortunately, in most of the existing current amplifiers of this kind, circuit structures are usually complex, involving amplifiers, multiple current sources and current mirrors. As explained with the first approach, current mirrors may be dysfunctional with weak currents. Furthermore, in many reported current amplifiers, a four-level cascoding structure is often used. In each cascoding structure, the transistors are usually set in a saturation mode. It may be difficult to meet such requirements if the supply voltage is low.
Consequently, there is a need for an efficient current amplifier operating at weak current levels.
It is another object of the present invention to provide a current amplifier which operates with low current signals, in the pico/nano ampere range.
It is an object of the present invention to provide a current amplifier which dissipates low power, e.g. in the range of 0.5 mW, while having a high current gain as well as a high gain-bandwidth product for signals which may be low current signals, in pico/nano ampere range.
Yet another aspect of the present invention is to provide such current amplifier which may be implemented on a small silicon surface, e.g. in CMOS.
According to one aspect of the invention, there is provided a method for converting a time-variable voltage signal into a time-variable output current signal using at least one transistor, the method comprising the steps of receiving the time-variable voltage signal; transmitting the time-variable voltage signal to the drain of a transistor, the transistor being maintained in a linear voltage-current operating mode; converting the time-variable voltage signal into a time-variable current signal using the transistor; and adding the time-variable current signal to a substantially fixed current signal generated by a substantially fixed current source to provide an output time-variable current signal.
According to another aspect of the invention, there is provided a converter for converting a time-variable voltage signal into a time-variable current signal, the converter comprising a first transistor for receiving the time-variable voltage signal and providing a shifted time-variable voltage signal; a second transistor which is maintained in a linear current-voltage operating zone, the second transistor receiving the shifted time-variable voltage signal and providing an intermediary time-variable current signal, the intermediary time-variable current signal being proportional to the time-variable voltage signal; a substantially fixed current source; an output, the output receiving a substantially fixed current signal from the substantially fixed current source and the intermediary time-variable current signal to provide a time-variable current signal, wherein the linear current-voltage operating zone of the second transistor and the substantially fixed current source ensures an efficient current amplification.